This invention relates to maintenance-free lead-acid cells, and more particularly to a maintenance-free cell with an improved separator and electrolyte distribution within the separator for improved shorting resistance, while accommodating gas transport in a normally sealed configuration.
Separators for maintenance-free cells having multiple layers, including highly retentive ultrafine fiber diameter glass materials, are known, for instance as taught in Eisenacher et al. in U.S. Pat. No. 3,753,784. The use of microporous layers sandwiched between the glass layers for resistance to penetration by metallic growths from the plates is also a known technique.
The subject invention is directed primarily to limited electrolyte maintenance-free cells and a multilayered penetration-resistant separator for use therein, wherein the separator contains the bulk of the electrolyte contained in the cell. The separator has an extremely high electrolyte retentivity and yet is thin with an overall dimension of less than about 1.5 millimeters. Such thin separators made of microfiber diameter glass are extremely non-homogeneous, for instance exhibiting variations from plus or minus ten percent in glass weight per unit area. Because of this extreme thinness in overall separator dimension coupled with the marked variations in structure of the separator layers, relatively high current densities will exist at the plates in the regions of lowest electrical resistance and lowest tortuosity. These regions develop bumps and exhibit shape change eventually leading to short-circuiting as the opposite polarity plates come into mutual contact.
It is a primary object of the subject invention to provide a highly electrolyte retentive thin separator for use in sealed maintenance-free calls, operating with internal gas recombination, by "homogenizing" the electrolytic resistivity and structure of the multi-layered separator, and by providing uniform electrolyte distribution and void volume distribution within the separator layers to enable long cell life in a normally sealed mode.
Additional relevant prior art includes U.S. Pat. No. 3,862,861 to McClelland et al, U.S. Pat. No. 3,900,341 to Shoichiro et al, Canadian Pat. No. 451,391 to Wells, and Japanese Pat. application Nos. SHO 38/1963-39288 and SHO 39/1964-30352.